Slips are used for various downhole tools, such as composite plugs and packers. The slips can have inserts or buttons to grip the inner wall of a casing or tubular. Examples of downhole tools with slips and inserts are disclosed in U.S. Pat. Nos. 6,976,534 and 8,047,279.
Inserts for slips on metallic and non-metallic tools must be able to engage with the casing to stop the tool from moving during its operation. On non-metallic tools, the inserts can cause the non-metallic slips to fail when increased loads are applied. Of course, when the slip fails, it disengages from the casing.
Inserts for slips are typically made from cast or forged metal, which is then machined and heat-treated to the proper engineering specifications according to conventional practices. When conventional inserts are used in non-metallic slips, they are arranged and oriented as shown in FIG. 1A. The slip 20 is disposed adjacent a mandrel 10 of a downhole tool, such as a composite plug, packer, or the like. The slip 20 moves away from the mandrel 10 and engages against a surrounding tubular or casing wall when the slip 20 and a cone 12 are moved toward one another. Either the slip 20 is pushed against the ramped surface of the cone 12, the cone 12 is pushed under the slip 20, or both.
FIG. 2A illustrates a side cross-section of a slip 20 having holes 22 for inserts according to the prior art, and FIG. 2B illustrates a side cross-section of the slip 20 with inserts 24 disposed in the holes 22. FIG. 2C illustrates a front view of the slip 20 with the holes 22 for the inserts. The slip 20 can have a semi-cylindrical shape. The holes 22 in the surface 21 of the slip 20 can be an array of blind pockets. The slip 20 can also have annular slots 26 for a tie strap or other retaining feature. The inserts 24 are anchor studs that load into the pockets 22 and can be held with a press fit or adhesive.
As shown in both FIGS. 1A and 2A, the pockets 22 and the inserts 24 disposed in those pockets 22 intersect the slip 20 at an acute bite angle β with respect to a line perpendicular to the slip's surface 21. Thus, the conventional arrangement places the inserts 24 at an angle β toward the ramped surface 13 of the cone 12 and the incline 23 of the slip 20. The angle β can be from 10 to 20-degrees, for example, so that the top face of the insert 20 is oriented at the same angle β relative to the top surface of the slip 20, as best seen in FIG. 2B.
By providing this angle β, the inserts 24 can better engage the casing wall. For example, when the slip 20 is fully extended to a set position against the casing wall, the inserts 24 inclined by the acute angle β present cutting edges with respect to the inside surface of the casing. With this arrangement, the inserts 24 can penetrate radially into the casing. Angled toward the cones 12, this penetration can provide a secure hold-down against pushing and pulling forces that may be applied through the tool's mandrel 10 and element system.
The arrangement of the inserts 24, however, can damage the slips 20 or the inserts 24 themselves. As shown in FIG. 1B, load on the cone 12 during use of the downhole tool can cause the inserts 24 to put stress on the slip 20. As a result, the slip 20 can fracture at the edges of the pockets 22 toward to the top surface 21 and bottom surfaces 27 and 23 of the slip 20. In another form of failure shown in FIG. 1B, shear forces on the inserts 24 can cause the exposed ends of the inserts 24 to shear off along the slip's top surface 21.
The inserts 24 may also be composed of carbide, which is a dense and heavy material. When the downhole tool having slips 20 with carbide inserts 24 are milled out of the casing, the inserts 24 tend to collect in the casing and are hard to float back to the surface. In fact, in horizontal wells, the carbide inserts may tend to collect at the heel of the horizontal section and cause potential problems for operations. Given that a well may have upwards of forty or fifty composite plugs used during operations that are later milled out, a considerable number of carbide inserts 24 may be left in the casing and difficult to remove from downhole.
The subject matter of the present disclosure is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.